Embodiments of the invention relate generally to centrifugal compressors. More specifically, embodiments of the invention relate to internally-cooled centrifugal compressors for improving compressor efficiency.
Compressors are well known in several industrial applications as machines having a primary function of increasing the pressure of a gas. Gas processed by a compressor is subject not only to pressure increase but also to temperature increase, due to heat developing in the gas when mechanical work is applied thereto for compressing the gas. Therefore, the gas temperature is considerably higher at the delivery side than at the suction side of the compressor. This is particularly the case when the compressor is a multistage compressor, including a plurality of sequentially arranged impellers, each provided with respective diffuser and return-channel. A multi-stage compressor achieves a high pressure ratio, which is linked with high temperature increase.
Due to the temperature increase, gas compression requires a large amount of power. In order to reduce the power required to achieve a certain pressure ratio, it is known to arrange so-called interstage coolers or intercoolers between one compression stage and the next. Intercooling reduces the density of the gas and the temperature thereof, removing heat from the gas delivered by one compressor stage before delivering the gas to the subsequent compressor stage.
The use of one or more interstage intercoolers improves the overall efficiency of the compressor. However, intercoolers are complex and cumbersome devices, which increase the footprint and overall dimension of the compressor and the cost thereof.
Moreover, the use of intercoolers requires complex piping to be arranged in order to have the gas flowing out of a compressor stage, through the intercooler and be again delivered at the inlet of the subsequent compressor stage.
In recent times, efforts have been made to design so-called internally cooled centrifugal compressors, which are simpler and more efficient. FIGS. 1A and 1B illustrate a known internally cooled centrifugal compressor of the current art.
More specifically, FIG. 1A illustrates a sectional schematic view of two sequentially arranged compressor stages 101, 102 of an internally cooled centrifugal compressor 100 of the current art and FIG. 1B illustrates an enlargement of the return-channel and diffuser of one of the compressor stages 101, 102. As shown in FIGS. 1A and 1B, compressor 100 comprises a shaft 105 arranged for rotation in a casing 107. Impellers 108, 109 are mounted for rotation on the shaft 105. A diffuser 110 is arranged at the outlet of impeller 108 and is fluidly coupled to a respective return-channel 111. The return-channel 111 is provided with return-channel blades or vanes 112 which connect an internal diaphragm portion 113 to an external diaphragm portion 114. The return-channel 111 is fluidly coupled to the inlet of the second impeller 109. A diffuser 115 is fluidly coupled to the outlet of the second impeller 109 and with a second return-channel 117 which can also be provided with return-channel blades 119 connecting the respective internal diaphragm portion 120 with the external diaphragm portion 114.
Gas entering the first impeller 180 is accelerated by the rotation of the impeller and subsequently slowed down in the diffuser 110, such that at least part of the kinetic energy imparted to the gas by the rotating impeller is converted into pressure energy. The partly pressurized gas is returned through return-channel 111 to the second impeller 109 for further acceleration. In diffuser 115 the accelerated gas delivered by the second impeller 109 is again slowed down and kinetic energy is partly converted into pressure energy and the gas is returned trough the return-channel 119 towards a further downstream compressor stage, not shown.
In order to improve the compressor efficiency a cooling channeling 123 is combined with the first compressor stage 101 and a second cooling channeling 124 is combined with the second compressor stage 102. As shown in the enlargement of FIG. 1B, according to the current art the channeling 123 and similarly the channeling 124 comprise a plurality of pipes extending from the external diaphragm portion through the return-channel blades 112, in the internal diaphragm portion 113 and back towards the external diaphragm portion. A coolant, for example, a liquid or a gas or a two phase fluid thus circulates through the internal diaphragm portion 113 and the blading 112, 119, to remove heat.
The efficiency of known heat removal systems integrated in the centrifugal compressors of the current art are not particularly efficient. According to known embodiments, the surface of heat exchange between the processed gas and the coolant
Thus a need exists for more efficient cooling arrangements, in order to improve the efficiency of internally cooled centrifugal compressors.